2008 Pontiac G6

We were driving the car and the temp gauge went to 0. The check engine light came on. We were close to Autozone and pulled in. He put a reader under the dash and it pulled up a P128 code and 1 fault code that he didn't know what that Fault code meant. He said I needed a thermostat and permatex. I wanted to check to see if this is what it might be. We placed coolant in it drove home and it did it again when I went to TSC. What is a fault code and what is the P128. I sincerely appreciate any assistance on this! Thank you, Bill Ohio

The only other code was a fault code 01/01. When he read the meter that plugged in under the dash, the P0128 goes away and the temp gauge goes to 150 and no higher. The meter reads p0128 and fault 01/01. Does this assist in anyway at all. My daughter just drove it to work and the temp gauge went back to zero, the temp gauge only goes to 150 & no higher. The coolant is full though and staying that way!!